Generates a css sprite map from the files matching the glob pattern. Uses the
keyword-style arguments passed in to control the placement.

Only PNG files can be made into css sprites at this time.

The $glob should be glob pattern relative to the images directory that specifies
what files will be in the css sprite. For example:

$icons: sprite-map("icons/*.png");
background: $icons;

This will generate a css sprite map and return a reference to it. It's important to
capture this to a variable, because you will need to use it later when creating
css sprites. In the above example you might end up with a new file named
images/sprites/icons-a2ef041.png and your css stylesheet will have:

The exact image name is not something you should depend on as it may change based on the
arguments you pass in. Instead, you can use the sprite-url() function to create a
reference to the css sprite map without generating the image again. Alternatively, simply
using the sprite map variable in an property will have the same effect as calling
sprite-url().

For each sprite in the css sprite map you can control the position, spacing, and whether or
not it repeats. You do this by passing arguments to this function that tell each sprite
how to behave. For instance if there is a icons/new.png then you can control it like so:

Passing true for the $use-percentages argument will return the
sprite position in percentages instead of pixels. This is useful if you
need to be able to scale the sprite up and down. Following the example
above, this: